If you want instant frags I would do as mentioned and chisel underneath taking some substrate with it. If you have time and want more frags you can cut through the the stock leaving a bit of the base behind which will regrow, and then take the piece you cut off, chop into several pieces and put them in a Tupperware type container in a very low flow area with some rubble or coarse sand in the bottom. Each piece will settle and regrow onto some of the rubble then remove after several weeks and glue to frag plug. Mushrooms are very easy to frag IME